The project objective is to advance current understanding of the organization of the medial nucleus of the trapezoid body (MNTB). The MNTB, present in the brainstem of all mammals including man, is a pivotal cell group in the pathway supplying centers involved in sound localization and efferent feedback to the cochlea. Damage to this region is associated with a loss of he ability to localize sound on the basis of interaural intensity cues. However, detailed knowledge of this cell group and its connections are limited as the MNTB lies deep within the passing fibers of the trapezoid body where it is virtually inaccessible to study by most current techniques. The study objectives are to be met with an in vitro brainstem slice preparation developed in a mammalian species with unusual metabolic tolerances, the brown bat: Eptesicus fuscus. The thick brainstem slices which can be maintained with this species allows the labeling, by intracellular dye injection, of large numbers of individual MNTB cells and long axonal processes with previously unattained detail. Our goals are (1) to lay the ground work for single cell analysis by characterizing the cytoarchitecture of the MNTB with Golgi- stained material from three bat species and the rat. (2) Intracellular labeling of cells and axons in the slice preparation will be used to characterize, by their axon collateral projections, the different classes of the MNTB principal cell the different classes of calyciferous axons. The main ending of these calyciferous axons represent the largest terminal specializations known in the mammalian brain, each enveloping a single MNTB principal cell. (3) The slice preparation will be used to determine the degree of target overlap between the collateral systems of specific pairs of principal cell and calyciferous axons, a morphological feature which may hold the key to their functional significance. The long-term goal of this project is to utilize these morphological data and the slice preparation to explore the cellular physiology of the different classes of anatomically defined MNTB principal cells and calyciferous endings.